1. Field of the Invention
The present invention relates to an image pickup apparatus using a solid state image pickup device.
2. Description of the Prior Art
In the case of using a solid state image pickup device, for example, a CCD (charge coupled device), in e.g., a television camera, it has conventionally been proposed that the accumulating time of the image pickup device can be reduced to less than the field (or frame) period of a television signal by changing the method of driving the solid state image pickup device.
For instance, in the U.S. patent application Ser. No. 596,404 filed on Apr. 3, 1984, there has been proposed by the assignee of this application the driving method whereby, for example, in solid state image pickup devices of the frame transfer type, the charges in the photosensing unit to perform the photoelectric conversion and the accumulation of the charges are drained by once vertically transferring them within the vertical interval and the remaining interval of the vertical interval is further used as the substantial accumulating time.
According to this method, in the television camera of, e.g., the NTSC system, although the accumulating time is ordinarly 1/60 second, this time can be set to 1/120 second, 1/500 second, or the like; therefore, there are effects such that the iris need not be reduced even in the case of a large amount of incident light, an image of an object moving at a high speed is not moved and the like. However, while the foregoing operation is being performed, in particular, if the substantial accumulating time is reduced when the charges which are accumulated for the period of time within the vertical interval are drained without taking them out as a signal as mentioned above, the amount of charges to be drained will have considerably increased. For example, when the substantial accumulating time (t.sub.1) is set to 1/500 second, the ratio of the accumulating time t.sub.2 of the charges to be drained to the time t.sub.1 becomes EQU t.sub.2 /t.sub.1 =(1/60 -1/500)/1/500.apprxeq.7.3
Namely, the ratio becomes about 7.3. Assuming that the level of charges which are accumulated for the period of time t.sub.1 is the normal level, charges which are 7.3 times larger than normal level are accumulated for the time t.sub.2. In the case where such a large number of charges are accumulated in this manner, it is very difficult to drain all of these charges and prevent their having an influence on the substantial accumulating time. Particularly, a great quantity of charges are generated in what is called a highlight portion or the like on the display screen and when the vertical transfer is performed within the vertical interval to drain the charges, a large amount of charges remain, so that an image which is displayed on the screen is adversely influenced.
In the output of the image pickup device, the amount of those residual charges becomes four to ten times (nearly the level of the saturation signal of the image pickup device) as large as the signal level. Thus, such a high signal level causes abnormal response of the signal processor and malfunction of the gain control or iris control.
In addition, an image pickup apparatus in which the charges in both the photosensing unit and the storage unit are drained within the vertical interval has also been proposed. According to this apparatus, no output signal is obtained for the interval before the transfer within the vertical interval, so that the iris control, gain control and the like become inaccurate.